1. Field of the Invention
This invention relates to a focusing lens position controlling apparatus for a variable focal length lens, and particularly to a focusing lens position controlling apparatus for an auto focus camera and an auto focus video camera containing a zoom lens of the rear focus type or the inner focus type therein.
2. Related Background Art
In recent years, it has been commonly practised to carry a zoom lens and an auto focusing function on a still camera or a video camera. If a zoom lens of the rear focus type or the inner focus type is adopted as a zoom lens carried on such a camera, it will become possible to make the focusing lens group thereof small and thus, the entire camera can be made compact, and this is preferable. However, the amount of axial movement of the focusing lens group of such a zoom lens differs not only depending on the object distance but also depending on the focal length and therefore, it has been impossible to obtain an accurate amount of axial movement by methods used in conventional auto focus cameras (for example, (1) a method of memorizing the corresponding table of the amount of axial movement of the lens corresponding to the object distance; or (2) a method of memorizing the relation between the object distance and the amount of axial movement of the lens as an operation expression; in this case, it is usual that the operation expression is a quadratic polynomial regarding the inverse number of the object distance). For example, methods of controlling the position of the focusing lens group of a zoom lens of the rear focus type or the inner focus type are proposed in Japanese Patent Application Laid-open No. 62-284317, Japanese Patent Application Laid-open No. 1-201634 (corresponding U.S. Pat. No. 4,914,464), Japanese Patent Application Laid-open No. 5-113533, etc.
However, the method disclosed in the aforementioned Japanese Patent Application Laid-open No. 62-284317 suffers from the disadvantage that it is necessary to memorize a curve prescribing the relation between the focal length and the amount of axial movement of the focusing lens, for a plurality of object distances, it is necessary to make the number of distance divisions great if an attempt is made to obtain high focusing accuracy also when the focal length of the zoom lens is extended, and an enormous memory capacity becomes necessary if an attempt is made to memorize the curve of the relation between the focal length and the amount of axial amount for respective photographing distances.
Also, in the method disclosed in the aforementioned U.S. Pat. No. 4,914,464, the relation between the object distance and the amount of axial movement at a reference focal length and the conversion factor for each focal length have been memorized, and at a certain focal length, a conversion coefficient corresponding to this focal length and the amount of axial movement at the reference focal length have first been found, and these have been calculated to thereby find the actual amount of axial movement of the focusing lens group. However, an attempt to make the number of divisions of the focal length greater with the tendency of the zoom lens toward higher magnification would give rise to the disadvantage that the number of conversion coefficients memorized for each focal length becomes greater and a large memory capacity becomes necessary.
Also, in the method disclosed in Japanese Patent Application Laid-open No. 5-113533, a specific coefficient for calculating the amount of axial movement for each focal length has been memorized, and the amount of axial movement of the focusing lens group has been found from this specific coefficient and the object distance by calculation. Again in this method, however, an attempt to make the number of divisions of the focal length greater with the tendency of the zoom lens toward higher magnification would give rise to the disadvantage that the number of specific coefficients memorized for each focal length becomes greater and a large memory capacity becomes necessary.